Nergal

In the DC Universe there are two creatures named Nergal. Both are deadly, devious, demonic and dead. But how did they meet their ends?

Thereby hangs a tale. Two, in fact, with one being slightly less tedious than the other:

NERGAL FLAVOUR ONE: THE DEMON

Nergal was born as a human in ancient Babylon. Little is known of his mortal life, except that he sexually abused children and was eventually burned as a witch. On dying, his soul descended to Hell, where ten thousand years* of toil and torture followed before he was initiated into demonhood. He eventually rose through the ranks of the damned to become the Arch-Duke of Mendacity. He may or may not be the Sumero-Babylonian deity of the same name.

Nergal first made his presence known in the 1940s, when he appeared on Earth and began sending out ghosts to terrorise mortals. Black Nergal (as he was known then) was defeated by Doctor Fate and sent packing back to Hell, where he plotted his return.
(More Fun Comics #67, "The Origin of Doctor Fate")

He later developed a more sinister agenda, persuading humans to murder each other for his amusement. It was after one of these killings that Nergal first met cynical conman and amateur occultist John Constantine. Constantine, who was only a child at the time, had no idea that Nergal was a demon, and accepted his first cigarette from the monster. Nergal was pleased to think that he had corrupted one so young - until he realised that John had stolen his cigarettes!
(Hellblazer Secret Files, "The First Time")

Nergal dropped off the supernatural radar for some time, until the late 1970s, when John Constantine and his friends came across a nasty case of possession in a club in Newcastle, England. Astra, the young daughter of the club's owner, had been subjected to ritual sexual abuse by her father and his friends for years. Her means of escape was to summon forth an "imagniary" demon named Norfulthing, which had slaughtered the club's patrons and possessed the child.

Rather than follow the advice of his friends, Constantine attempted to summon an even more powerful demon to destroy Norfulthing. He succeeded in bringing forth Nergal, but failed to properly name him, and in accordance with supernatural law (lore?), lost all power over the monster. Nergal destroyed Norfulthing anyway, but demanded the girl's soul as payment. Constantine offered up his own soul instead, but Nergal refused it. As the jaws of Hell began to close on Astra, Constantine tried to drag her to safety, but only succeeded in freeing her (now severed) arm. With guilt lying heavy on his soul, Constantine fell into the depths of insanity and was locked up in Ravenscar Mental Asylum.
(Hellblazer #11, "Newcastle: A Taste of Things to Come")

Constantine and Nergal would cross paths again ten years later, when the magician's investigations into a series of brutal murders and strange crimes brought him into contact with a group known as "The Damnation Army". Led by Nergal, the Army was an underground militia of black magicians, psychopaths and demonic thralls who seemed to be plotting the destruction of society. Constantine, however, unearthed a second aim: the complete annihilation of "The Resurrection Crusade", a group hoping to create a new holy saviour.
(Hellblazer #6, "Extreme Prejudice")

It seems that a stone tablet unearthed from Hell had prophecised the birth of a creature that could become a great healing force for the world. Realising that the Resurrection Crusade's plan would result in the creation of this creature, Nergal began constructing his Damnation Army as an opposing force. When he became aware of Constantine's presence, however, a new plan came to mind. Nergal appeared to a horribly crippled Constantine and threatened to devour a maternity ward full of newborn babies unless the magician would agree to work for him. With no choice but to yield, Constantine recieved a dose of demonic blood from Nergal which instantly healed him, but left the two supernaturally connected.
(Hellblazer #8, "Intensive Care")

Constantine, manipulated by Nergal into destroying The Resurrection Crusade's new messiah, now realised that the balance between Heaven and Hell had been lost. In order to stop Hell from staking its claim on the Earth, the Hell stone's prophecy needed to be fulfilled; a healing force had to be conceived. Constantine thus allowed his body to be possessed by a plant elemental called the Swamp Thing, who used it to impregnate his human lover Abigail Arcane. Whilst they made love, Constantine's soul was running through various astral planes to avoid Nergal. Only a last minute interjection from a post-coital Swamp Thing stopped Constantine from a horrible demise. Swampy and Abby retired to their home in Louisiana, whilst Constantine returned to Newcastle where he began preparing for his final conflict with Nergal.
(Swamp Thing #76, "L'Adoration de la Terre" and Hellblazer #10, "Sex and Death")

Back in Newcastle, the place where their conflict began, Constantine used the skills of a computer magician to project himself into the electrical plane. Nergal attempted to follow him, but with Constantine's friend tripping all the right switches, the demon never had a chance; before he knew it, Constantine had led him right up to the borders of Heaven, where a somewhat less-than-amused angel dispensed with the demon in bloody fashion.
(Hellblazer #12, "Better the Devil You Know...")

This was not the end of Nergal, however; he continued to brood down in Hell, even creating his own city named Mashkan-Shapir. This was destroyed during a confrontation with Anton Arcane, Abigail's evil uncle-turned-demon.
(Swamp Thing #97, "Scattered Houses" and #98, "Family Reunion")

Nergal remained quiet for some time until The First of the Fallen decided to lay claim to Constantine's soul. Nergal objected, so the First pulled rank and turned the demon back into a human soul before ripping off his arms. Nergal was left to regain his demonic status. And limbs.
(Hellblazer #60, "Guys and Dolls, Part 2: Nativity Infernal")

Many years later, having lost his memory foiling yet another Apocalypse, John Constantine found himself suffering a sudden flashback in which friends and foes of times past, including Nergal, were glimpsed. Soon after, a strange girl with supernatural powers framed Constantine for murder, sending him on the run.
(Hellblazer #194, "Ward 24")

As the amnesiac Constantine fled to the coast, his niece Gemma made an attempt to psychically contact him, but the girl - now calling herself Rosacarnis - blocked her contact.
(Hellblazer #195, "Out of Season, Part 1")

Still stuck on the coast, Constantine found himself contacted by Rosacarnis, who now took on the appearance of a regal woman. She offered to return his memories if he would serve her for one day. He might not have had his memory, but Constantine still knew enough about demons to turn her down flat.
(Hellblazer #196, "Out of Season, Part 2")

With his "holiday" over, John returned to London as a homeless tramp. Once again, he found himself plagued by Rosacarnis, who took the form of a maggot-infested child's corpse. Fleeing her, John took up sanctuary in an old church run by a strange cult.
(Hellblazer #197, "Stations of the Cross, Part 1")

Of course, for Constantine sanctuary is often brief, and after pissing off the cultists he found himself tied to a stake and about to become a brief but noisy bonfire. Rosacarnis appeared to John and once again requested a day in her service in exchange for his memories and life. With the flames climbing ever higher, Constantine accepted and found himself vanishing away to nothing...
(Hellblazer #199, "Stations of the Cross, Part 3")

Having taking Rosacarnis' offer, John found himself reliving three alternate lives with three women from his past: Zed, Kit Ryan and Angie Spatchcock. But something was wrong each time; details were missing, people were acting out of character and each one of his three children was in some way connected to a series of brutal murders and strange disappearances.
Eventually the truth was revealed: Rosacarnis had used a kind of temporal magic to bend time, creating three alternate lives during which she - disguised as Zed, Kit and Angie - could sleep with John and bear his children.
To keep up her end of the bargain, she returned Constantine's memories. All of them, including the false ones he'd experienced in her alternate realities. And why? Because she wanted to make John's life Hell. Because she wanted to pay him back. Because she was the daughter of Nergal!
(Hellblazer #200, "Happy Families")

As John stumbled back into his old life, unable to tell which of his memories were real, his children began to mercilessly slaughter his friends and acquaintences. After an attack which killed Albert and hospitalised Clarice, John realised what was going on and ran to help his best friend Chas - only to arrive just as a petrol tanker crashed into the taxi rank he was stood in.
(Hellblazer #202, "Reasons to be Cheerful, Part 1")

Still, you can't keep a good taxi driver down - especially not when there's something supernatural keeping him alive. A mysterious creature in the form of a rat opened up a tunnel into the fire for Constantine, who crawled into the remains of the building to find Chas miraculously alive. The celebrations were soon quashed, however, when the rat dived into Chas' chest, taking over his body.
The Entity controlling Chas explained to John that his children, unable to kill their father for fear of undoing the spell that created them, had instead settled for destroying his life by slaughtering his friends. With time ticking down, John and The Entity took to the road to save as many as they could...
(Hellblazer #203, "Reasons to be Cheerful, Part 2")

John and The Entity managed to rescue Angie, John's girlfriend. But whilst they were distracted with her, the children took the opportunity to hypnotise the boyfriend of John's niece, Gemma, into plotting her death.
(Hellblazer #204, "Reasons to be Cheerful, Part 3")

John and The Entity were too late to save Gemma, so it's a good job she was smart enough to save herself. After she'd broken the kneecaps of her dipshit boyfriend (try it, ladies!), it seemed like everyone was safe. But Gemma was just a distraction from the real target: John's sister. With his sister dead and John reeling, The Entity decided to exit Chas' body and reveal its true identity: Nergal!
(Hellblazer #205, "Reasons to be Cheerful, Part 4")

Chas, infuriated that John had dragged him into another game of life and death, stomped out of the flat. In a black mood, he then proceeded to screw a teenage prostitute before beating his wife, Renee. Out drinking, he met John's demonic daughter, Maria, who coerced him into killing two men. Sensing something odd within him, she convinced him to take her back to his house, where he inadvertantly made her confont her own fictional past (which she experienced in the pocket universe her mother had trapped John in). As payment, she reached into Chas' soul and pulled out the thing that had been making him uncontrollably angry - a small piece of Nergal that the demon had left behind. Maria left Chas to pick up the pieces of his life, warning him that Nergal's taint only took away his inhibitions - the violence he had enacted was all Chas' own.
(Hellblazer #206, "Cross Purpose")

NERGAL FLAVOUR TWO: THE GOD

Unlike his namesake, this Nergal was not actually a supernatural being at all, but a member of a spacefaring people from Oa, the planet from which the power of the Green Lanterns stems. Nergal and his group settled down on Earth millennia ago, using their powers to act as gods in ancient Babylon and Sumeria. Nergal immediately set about terrorising the populace, quickly becoming known as the God of Death. He and his kin also attacked the Amazons and Atlantis during their early years. Nergal was eventually defeated by Ishtar, the Goddess of War, and condemned to Kurnugi, a kind of pseudo-Hell, for many thousands of years.
(Green Lantern Annual #9, "Mother of Heaven, Part 2: Lady of Battle")

Being the badass that he was, Nergal eventually figured out a way to escape the Hell dimension. He and his cronies kidnapped both the Green Lantern and the archaeological team of Sala Nisba, a descendant of Ishtar and possessor of her powers. The demon hoped that by combining the powers of Sala/Ishtar's staff and the Green Lantern's ring, he could open up the portal between Kurnugi and Earth and gain his freedom. The portal was opened, but Sala was able to hold back Nergal and his troops whilst the Green Lantern rescued Sala's associates. The portal collapsed just behind the Lantern, leaving Sala trapped with Nergal and his warriors in Kurnugi.

Realising that his friend was in great peril, the Lantern gathered together Batman, Superman, Wonder Woman, Aquaman and Doctor Fate and plotted to storm Kurnugi. Doctor Fate, weakened by the power used to open the portal, was unable to follow the team as they entered the Hell dimension.
(JLA: Gatekeeper #1)

Nergal's monstrous hordes made a swift and brutal attack, giving the JLA have a hard time of it - especially Superman, whose powers were adversely affected by the Hell planet's twin suns. The monsters actually managed to gain the upper hand until Sala and her newly mustered army of gnomes came to the JLA's rescue. No, really. Having accepted her role as the heir to Ishtar's powers, Sala refused to return to Earth with the Green Lantern until Nergal and his warriors were defeated. The ever-heroic JLA offered their support.

Nergal soon rallied around and sent his troops into battle again, choosing to lead them himself this time. During this battle Ishtar's ringstaff was destroyed, taking the portal back to Earth with it. Nergal was driven back, whilst the JLA discovered that their only hope of getting home was to find five objects which would either create a portal back home or become the deadliest weapon ever made. Natch.
(JLA: Gatekeeper #2)

Having succeeded in acquiring the five powerful objects, the JLA prepared for their return to Earth - only to find that one final ingredient, the blood of an immortal, was missing. Nergal's troops arrived again, but their attack was halted by Milissu, Nergal's sort-of-step-mother who tricked him into spilling blood. This activated the portal spell, destroying Nergal and his blood relatives in the process. C'est la vie. Though not for Nergal, obviously.
(JLA: Gatekeeper #3)

*time is much more fluid in the underworld; a single day on Earth can seem like a thousand years of torture in Hell. For this reason it's difficult to figure out exactly how long it took for Nergal to develop from human soul into full-blooded demon.